Portable devices routinely depend upon batteries as a power source. To ease battery replacement costs, rechargeable batteries have found wide utility in powering contemporary consumer and business products. For example, nickel cadmium batteries may be repeatedly used to energize computers, radios, pagers and other such devices.
An important element of any reliable battery charging system is the battery charging unit. Ideally, a battery charging unit should maintain a constant charging current regardless of any variations in the load current of the battery being charged. While this may not be difficult for static battery chargers (i.e., stationary battery chargers charging batteries not contemporaneously powering an operating device), maintenance of a constant charging current for a vehicular charging unit proves quite difficult since the device under power may draw varying amounts of current. For example, a two-way radio may draw a large amount of current when transmitting, and significantly less current while receiving. Further, lesser variations in load current may exist while the device is operating in a single mode. Thus, a two-way radio in the receive mode may draw varying amounts of current depending upon the speaker volume setting or other such operator adjusted controls.
Historically, vehicular battery charging units have operated either to make gross current changes (i.e., from trickle charge to rapid charge) responsive to manual controls on the charging unit itself, or to maintain a fixed charger current regardless of the load current of the device. This later approach is particularly undesirable since maintenance of a fixed charger current greater than the ideal charging current may result in overcharging and overheating the battery, which typically results in reduced battery life. Conversely, maintenance of a low charger current, for example, a continuous trickle charge, may result in insufficient charging of the battery or, in certain circumstances, further discharging the battery to supply the required current load. Accordingly, a need exists in the art for a battery charging unit that maintains an optimum battery charging current, regardless of battery load current, thereby maximizing battery life.